JP2632726B2 - Fuel assembly for boiling water reactor - Google Patents

Description

The present invention relates to a fuel assembly for a boiling water reactor (BWR), and more particularly, to a fuel assembly for a uranium dioxide fuel rod and a plutonium-rich fuel rod. It relates to the effective use of plutonium in mixed fuel assemblies.

[Prior Art] FIG. 4 shows a conventional 8 × BWR.
FIG. 2 is a cross-sectional view showing a state in which the fuel assembly 8 is loaded on the core.

In the figure, an 8 × 8 type fuel assembly generally denoted by reference numeral 80 has a total of 62 fuel rods 81 and two water rods (hollow tubular members) which are hollow and do not contain a fuel substance and flow cooling water. Elements) wr are arranged in a square grid of 8 rows and 8 columns, and the outer periphery thereof is surrounded by a channel box 10 made of Zircaloy. Here, the nuclear fuel loaded into the fuel rod 81 will be described later.

The 8 × 8 type fuel assembly 80 configured as described above is loaded with two sides adjacent to a control plate (control blade) 1a in which a neutron absorbing material is loaded as shown in the figure.

During the power operation of the nuclear reactor, the cooling water also serving as a moderator flows from the lower part of the fuel to the upper part thereof, and removes the heat generated from the fuel rod 81. At this time, no void is generated in the bypass region (water gap portion where the control rod 1 and the instrumentation pipe i are arranged) B outside the channel box 10, but the channel box 10
Voids are generated in the cooling water flow path (in-channel region) I inside the inside.

Accordingly, the density distribution of the cooling water (moderator) in the cross section as shown in FIG. 4 is not uniform, and the density is high in the bypass region B and low in the in-channel region I. Therefore, the thermal neutron flux distribution in the cross section of the fuel assembly 80 is not uniform. That is, the thermal neutron flux is high in the periphery of the aggregate facing the bypass region B having a high moderator density, and the thermal neutron flux is low and the average energy of neutrons is high in the central portion of the aggregate having a low moderator density. I have.

Next, a conventional example in which a plutonium-enriched fuel is loaded on the 8 × 8 type fuel 80 as described above will be described. In the following description, a uranium-plutonium mixed oxide (MOX) fuel is used as an example of the plutonium-enriched fuel.

The 8 × 8 type fuel 80 in FIG. 4 also shows a fuel-only arrangement of an Atoll type fuel assembly disclosed in, for example, Japanese Patent Application Laid-Open No. 60-242391.

In the fuel rods 81 in FIG. 4, a symbol P in a circle indicates a fuel rod loaded with MOX fuel as a nuclear fuel (hereinafter, referred to as “MOX nuclear fuel”), and similarly, a symbol U
Indicates fuel rods loaded with uranium dioxide (UO ₂ ) fuel that mainly uses low-enriched uranium as nuclear fuel (hereinafter referred to as “UO ₂ fuel rods”).

In the example of the Atoll type fuel assembly 80, a total of 32 MOX fuel rods are arranged at a relatively peripheral portion of the assembly, and a total of 30 UO ₂ fuel rods are arranged in other regions (relatively central portion of the assembly). Fuel rods are located. Therefore, the ratio of MOX fuel rods to the total fuel rods per assembly is (32/62) × 100% = 52%, which accounts for about half.

As another conventional example, MOX fuel rods are arranged only in a part of the center of the assembly, and UO ₂
There are known island-type fuel assemblies in which fuel rods are arranged, and discrete-type fuel assemblies in which all fuel rods are MOX fuel rods.

[Problems to be Solved by the Invention] Among the above conventional examples, in the case of island-type fuel, MO
Since the arrangement position of the X fuel rod is far from the control rod, it is possible to prevent a reduction in the reactivity control effect (control rod value) of the control rod described below. However, since the MOX fuel rods are arranged only in a part of the center of the assembly, the loading amount of plutonium per assembly is reduced. Therefore,
It does not always meet the recent demand for the use of pull thermals, which intends to actively use plutonium in thermal neutron reactors.

On the other hand, in the case of the discrete type fuel and the atoll type fuel 80, although a large amount of plutonium can be loaded, the value of the control rod is reduced because the MOX fuel rod is located near the control rod. This is because plutonium has a larger thermal neutron absorption cross section and a smaller thermal neutron flux than uranium.

Further, in the case of the island type fuel and the discrete type fuel, the MOX fuel rods are arranged in the central part of the assembly. However, as described in the related art, the average energy of neutrons in the central part of the aggregate having a low moderator density is high, and Such an arrangement is not preferred for effective use of plutonium because of the poor economics.

The present invention has been made in view of the above-mentioned problems of the related art, and an object of the present invention is to provide a BWR fuel assembly that can effectively use plutonium without deteriorating neutron economics. It is. Also this B
It is also a part of the subject of the present invention to prevent a reduction in control rod value in a WR fuel assembly.

Means for Solving the Problems A fuel assembly for a BWR according to the present invention comprises a hollow tubular element filled with a moderator, and a plurality of first fuels loaded with a plutonium-enriched fuel as an unburned nuclear fuel. A rod and a plurality of second fuel rods loaded with uranium dioxide fuel as unburned nuclear fuel arranged in a square grid of 9 rows and 9 columns, the outer periphery of which is surrounded by a channel box; As an element, one or more large-diameter water rods corresponding to a plurality of fuels in the center of the array, or a large-diameter hollow tubular element such as a large-diameter water channel, and adjacent to the channel box in the array The first fuel rod is disposed at a peripheral portion to be formed and an inner peripheral portion adjacent to the large-diameter hollow tubular element, and the second fuel rod is disposed at another portion.
The above object has been achieved by disposing the fuel rods.

In this case, among the first fuel rods arranged in the peripheral portion, the first fuel rods are arranged in rows or columns facing the control plate of the cross-shaped control rod inserted outside the channel box in the peripheral portion. A configuration in which part or all of the first fuel rod is replaced with the second fuel rod may be adopted.

The plutonium-enriched fuel loaded on the first fuel rod is typically a uranium-plutonium mixed oxide fuel.

The term "unburned nuclear fuel" refers to the nuclear fuel loaded at the time of manufacturing the first and second fuel rods, and the type of fissile material generated by the burning of these fuel rods is not described. , Is not particularly limited. Addition of burnable poisons to nuclear fuel is also optional.

[Operation] Nuclear characteristics of uranium and plutonium Prior to the description of the operation of the present invention, the nuclear characteristics of UO ₂ fuel and MOX fuel will be described.

FIG. 5 is a graph showing the characteristics of the isotopes contained in the UO ₂ fuel and the MOX fuel.

In the figure, the horizontal axis of each graph (a), (b), (c) is the neutron energy, and the vertical axis is the fission cross section, absorption cross section, and the number of neutrons generated per unit neutron absorption (η), respectively. .

Of the plutonium isotopes contained in MOX fuel, fissile materials that cause fission by thermal neutrons are ²³⁹ Pu and ²⁴¹ Pu.
However, as is clear from FIG. 5, the fission cross section and η value of ²³⁹ Pu are larger than those of ²³⁵ U contained in UO ₂ fuel. Therefore, to obtain the (fission amount in other words is the same) to experience burning of the burnup in the core Compare fuel comrades in optimum deceleration state, MOX fuel, a large infinite multiplication factor from UO ₂ fuel There is an advantage that it can be.

On the other hand, ²⁴⁰ Pu does not cause fission by thermal neutrons, and, as is evident from the graph (b) of FIG.
"Resonant absorption energy region").

Also, since ²³⁹ Pu has a resonance absorption energy region near 0.3 eV, the η value of ²³⁹ Pu drops sharply there.

9 × 9 Fuel Assembly Next, the model of the fuel assembly to which the present invention is applied and its thermal neutron flux will be described.

In recent years, high burn-up fuels (for example, average take-up burn-up from 30 to 32 Gwd / t to 45 G
The development of a 9 × 9 fuel assembly has been promoted. FIG. 6 shows a configuration example of the cross section of the 9 × 9 type fuel. In this type of 9 × 9 fuel, the number of fuel rods is increased as compared with the above-mentioned 8 × 8 fuel 80, so that the average linear output density per fuel rod can be reduced.
In addition, the resulting margin can be used to increase the area of the non-boiling portion. That is, it is possible to increase the diameter of the hollow tubular element through which non-boiling water passes. For example, in a 9 × 9 type fuel 90 shown in FIG. 6, a square water channel W occupying a 3 × 3 region in the center of the assembly is provided in addition to a total of 72 fuel cells 91.
Is adopted. In addition, there has been proposed a configuration example in which one or a plurality of large-diameter water rods obtained by increasing the diameter of the water rod wr are provided.

In the following invention, a 9 × 9 type fuel simply means a 9 × 9 type fuel having a large-diameter hollow tubular element of this type, and a large-diameter hollow tubular element is a water channel. W will be described as a representative.

By the way, in the 9 × 9 type fuel, the water density at the center of the assembly can be increased by adopting the water channel W,
As a result, the thermal neutron flux distribution is also improved. As an example, FIG. 7 shows the thermal neutron flux distribution in the AA 'direction of the cross section of the assembly in FIG. 6. As shown in FIG. In a region around W, thermal neutrons are relatively high, and in a region sandwiched between them, the thermal neutron flux is relatively low.

The fuel assembly according to the present invention focuses on the above-described nuclear properties of uranium and plutonium and the thermal neutron flux distribution in 9 × 9 type fuel, and considers the first and second fuel rods. Determine the placement position. That is, MOX fuel rods (first fuel rods) are arranged in two regions, that is, the outer periphery of the high thermal neutron assembly and the periphery of the water channel W. In the low neutron flux region
Place UO ₂ fuel rod (second fuel rod).

Considering the neutrons absorbed by the MOX fuel rod under this arrangement condition, the proportion of the neutrons that are decelerated to the following energy ranges (A) and (B) becomes large.

(A) An energy region lower than the resonance absorption energy region.

(B) An energy region where the η value of ²³⁹ Pu takes a minimum and is even lower than the energy region smaller than the η value of ²³⁵ U.

Therefore, MOX fuel rods under the above arrangement conditions are ²³⁹ Pu and
²⁴⁰ Pu resonance absorption can be escaped, and ²³⁹ Pu η
You can take advantage of the fact that the value is greater than that of ²³⁵ U.

On the other hand, the arrangement area of the UO ₂ fuel rod under the above arrangement conditions is M
Not suitable for placing OX fuel rods. This is because the thermal neutron flux in this region is low and the neutron energy is high, so the MOX fuel lacks moderator.
Therefore, in the present invention, UO ₂ fuel rods are arranged in this area.

As described above, the neutron economy can be improved by arranging the MOX fuel rod in the region where the thermal neutron flux is relatively high and arranging the UO ₂ fuel rod in the region where the thermal neutron flux is relatively low.

In this case, the MOX fuel rods are arranged both on the periphery of the assembly and around the water channel, so that the plutonium loading per fuel assembly can be increased.

However, when MOX fuel rods are arranged at the portion of the control rod 1 around the assembly facing the control plate 1a (corresponding to the first row and first column shown in FIG. 6), the thermal neutron absorption cross section of plutonium becomes Because it is larger than uranium, the thermal neutron flux becomes smaller. Therefore, the value of the control rod is slightly reduced as compared with the case where the UO ₂ fuel rod is arranged at this position. As a configuration for taking this measure, a part or all of the MOX fuel rods arranged at the portion facing the control plate 1a are replaced with UO ₂ fuel rods. Thereby, a decrease in control rod value can be suppressed.

By the way, in the above explanation, plutonium-enriched fuel is
Although described using the X fuel as a representative, the characteristics of the MOX fuel with respect to the UO ₂ fuel are due to the difference in the nuclear properties of plutonium and uranium as described above. Therefore, plutonium-enriched fuels other than MOX fuel also have the same characteristics as above. Therefore, the first fuel rod in the present invention is not limited to the MOX fuel rod, and other types of plutonium-enriched fuel can be used.

Examples Example A FIG. 1 shows a design example of a 9 × 9 fuel assembly according to an example of the present invention.

In the figure, the schematic configuration of the 9 × 9 fuel, generally indicated by reference numeral 90A, is the same as that of the 9 × 9 fuel 90 shown in FIG.

However, the 32 fuel rods 91 arranged in the region around the assembly and the 16 fuel rods 91 arranged in the region around the water channel W are all MOX fuel rods. On the other hand, the 24 fuel rods 91 arranged between these two regions are UO ₂ fuel rods.

MOX in all fuel rods per 90A of 9x9 type fuel
The ratio of fuel rods is as high as {(32 + 16) / 72} x 100% = 67%. Therefore, conventional island-type fuel,
Alternatively, the loading amount of plutonium can be greatly increased as compared with the atoll type fuel 80.

Embodiment B FIG. 2 shows a 9 × 9 structure according to another embodiment of the present invention.
An example of the design of a fuel assembly is shown.

The 9 × 9 type fuel 90B shown in the figure has the fuel rods 91 arranged in rows and columns facing the control plate 1a of the control rods 1 at the periphery of the assembly, except for two corner rods C1 and C2. UO ₂ fuel rod.

As described in the operation section, it is possible to suppress a decrease in control rod value by adopting the configuration as shown in FIG. FIG. 3 shows control rod values in the design examples of the above-described embodiments A and B for comparison.

In the figure, the solid line A indicates Example A (9 × 9 type fuel 90A).
The dotted line B is the control rod value of Example B (9 × 9 type fuel 90B). When these Examples A and B are compared, Example A is more advantageous in terms of the amount of plutonium loaded per aggregate, but Example B is more advantageous in terms of control rod value.

By the way, in each of the above embodiments, one 3 × 3 water channel W is shown as a large-diameter hollow tubular element surrounded by a MOX fuel rod at the center of the assembly, but one or more water channels W may be used instead. May be used.

[Effects of the Invention] As described above, according to the BWR fuel assembly according to the present invention, the plutonium-enriched fuel rod is provided around the fuel assembly having a relatively high thermal neutron flux and around the large-diameter hollow tubular element. By arranging, plutonium can be effectively used without deteriorating neutron economy.

On the other hand, by adopting a configuration in which the plutonium-enriched fuel rods arranged in rows or columns facing the control rods are replaced with uranium fuel rods, it is possible to suppress a reduction in control rod value accompanying the use of the plutonium-enriched fuel rods. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a 9 × 9 fuel according to one embodiment of the present invention, FIG. 2 is a cross-sectional view of a 9 × 9 fuel according to another embodiment, and FIG. FIG. 4 is a cross-sectional view showing a state in which an 8 × 8 type fuel as a conventional Atoll type fuel is loaded in a core, and FIGS. 5 (a), (b), ( c) is a diagram showing the relationship between the neutron energy of uranium and plutonium, the fission cross section, the absorption cross section, and the number of neutrons generated per unit neutron absorption (η). FIG. 6 shows the 9 × 9 type to which the present invention is applied. FIG. 7 is a cross-sectional view for explaining the structure of the fuel, and FIG. 7 is a diagram showing the thermal neutron flux distribution of the preceding figure. [Description of Signs of Main Part] 1 Cross-shaped control rod 1a Control plate 10 Channel box 90A, 90B 9 × 9 fuel 91 Fuel rod P MOX fuel rod (first U: UO ₂ fuel rod (second fuel rod) W: 3 × 3 type water channel (large-diameter hollow tubular element) In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (3)

(57) [Claims] 1. A hollow tubular element filled with a moderator, a plurality of first fuel cells loaded with plutonium-enriched fuel as unburned nuclear fuel, and uranium dioxide fuel loaded as unburned nuclear fuel. 9 and the second fuel rods
In a fuel assembly arranged in a square lattice with rows and nine columns and having an outer periphery surrounded by a channel box, as the hollow tubular element, one or a plurality of fuel rods corresponding to a plurality of fuel rods at the center of the arrangement are provided. A large-diameter hollow tubular element, wherein the first fuel rod is disposed at a peripheral portion adjacent to the channel box in the array and at an inner peripheral portion adjacent to the large-diameter hollow tubular element; A fuel assembly for a boiling water reactor, wherein the second fuel rod is arranged. 2. A fuel assembly for a boiling water reactor according to claim 1, wherein, among the first fuel rods arranged at the peripheral portion, a cross-shaped control member inserted outside the channel box. A first arranged in a row or column facing the control plate
A fuel assembly for a boiling water reactor, wherein a part or the whole of the fuel rod of (1) is replaced by the second fuel rod. 3. The boiling water reactor according to claim 1, wherein the plutonium-enriched fuel loaded in the first fuel rod is a uranium-plutonium mixed oxide fuel. Fuel assembly.